Nontypeable Haemophilus influenzae (NTHI) is an important cause of otitis media in children and lower respiratory tract infection in adults with chronic obstructive pulmonary disease (COPD). A vaccine to prevent otitis media in children and lower respiratory tract infections in adults with COPD would reduce mortality (in case of COPD) and morbidity, improve quality of life and reduce health care costs. The development of such a vaccine is one of the goals of the work proposed. The antigenic structure of two important outer membrane proteins of NTHI (P6 and P2) will be elucidated and their potential as vaccine antigens will be assessed. P6 has several characteristics suggesting that it will be an effective vaccine antigen. In Specific Aim 1, the structure of P6 as it exists in the outer membrane will be elucidated and the human immune response following immunization with P6 will be characterized with emphasis on identifying protective epitopes recognized by human antibodies. P2 is the major protein in the outer membrane. Antibodies to conserved regions of P2 are highly bactericidal for multiple strains of NTHI, indicating that these conserved regions may form the basis of an effective vaccine. In Specific Aim 2, the sequence heterogeneity of two relatively conserved surface loops will be determined, antibodies to conserved loops will be developed and characterized, the stability of P2 in the human respiratory tract will be assessed, and broadly cross-reactive human bactericidal antibodies will be studied for binding to conserved loops of P2. The proposed work will advance the area of vaccine development for NTHI by elucidating the antigenic structure of two important surface proteins and rigorously assessing their potential as vaccine antigens with emphasis on elucidating protective immune responses in humans.